Orifice assembly for spinning low viscosity melts

ABSTRACT

AN IMPROVED ORIFICE ASSEMBLY AND PROCESS IS PROVIDED WHEREBY FINE DIAMETER AND/OR LIGHT FIBERS AND FILAMENTS MAY BE FORMED FROM ESSENTIALLY INVISCID MELTS WITHOUT ATTENDING SINUOUS EFFECTS UPON THE EXTRUDED MOLTEN STREAM.

Oct. 19, 1971 w. MOTTERN ETAL 3,613,158

ORIFICE ASSEMBLY FOR SPINNING LOW VISCOSITY MELTS Filed Dec. 15, 1969FIG.3.

INVENTORS JOHN W. MOTTERN ROBERT E. CUNNINGHAM ROBERT P. BELL FIG. 2.

ATTORNEY United States Patent US. (ll. l8-8 QM 4 Claims ABSTRACT OF THEDISCLOSURE An improved orifice assembly and process is provided wherebyfine diameter and/or light fibers and filaments may be formed fromessentially inviscid melts without attending sinuous efiects upon theextruded molten stream.

COPENDI'NG APPLICATIONS The subject matter of this application isrelated to the subject matter of commonly assigned and copendingapplications Ser. No. 829,216 filed June 2, 1969, of S. A. Dunn, L. F.Rakestraw, and R. -E. Cunningham and Ser. No. 838,593 filed July 2,1969, of R. S. Otstot and J. W. Mottern.

BACKGROUND OF INVENTION Field of the invention This invention relates toimprovements in the formations of fibers and filaments of materialswhich are essentially inviscid in the melt by extrusion of the moltenmaterial as a free jet into atmospheres which stabilize the nascentmolten fiber or filament prior to breakup caused by surface tensionpending solidification.

Discussion of the prior art and problems The recent advent of the filmstabilization of inviscid jets has been recognized as a practical meansfor forming filaments and fibers from materials which exhibit extremelylow viscosities in the liquid or molten phase. Application Ser. No.829,216 incorporated by way of reference herein, describes in detail andclaims various techniques by which stabilization may be established.

Briefly, where materials such as metals, metal alloys, and ceramicsexhibit extremely low viscosities in the molten state of less than about10 poises and more commonly only a fraction of a poise, the surfacetension of such a free molten filamentary stream is so great in relationto its viscosity that the stream tends to break into small spheres orshot before it can be solidified by cooling or quenching by practicalmeans. It has been discovered that the length of the molten inviscidstream or jet, or the time in which such a jet exists as a continuousstream prior to break-up due to its surface tension when extruded atappropriate velocities may be considerably increased by extruding theinviscid jet into an atmosphere which upon contact with the nascentmolten jet forms a thin film on the surface thereof. The stabilizingfilm must, of course, be rapidly formed, be a solid or at least have aviscosity substantially greater than that of the molten jet at thespinning temperatures employed. The film must also be substantiallyinsoluble in the molten jet under the spinning conditions so thatsubstantial, and desirably complete, continuity of the film is achievedand maintained.

The means for film stabilizing inviscid jets are known and are varied asare the materials which may be stabilized. For example, the molteninviscid jet may be extruded into atmopsheres which readily react withthe surface of the molten jet to form a film or into atmospheres whichdecompose upon contact with the molten jet to 3,613,158 Patented Oct.19, 1971 form films. Thus, a molten aluminum jet which is extruded intoan atmosphere containing oxygen is stabilized by the rapid formation ofaluminum oxide which is both solid at the optimum extrusion temperatureand is substantially insoluble in the molten jet. Aluminum oxide jets,on the other hand, may be extruded into hydrocarbon atmospheres, such aspropane, which upon contact with the hot ceramic jet decompose leaving astabilizing carbon film on the jet. In a special case described in US.Patent 3,216,076 issued to Alber et al., it was noted that the oxides ofcertain metals, such as iron, silver, and gold, are soluble in theirrespective melts to the extent that they do not serve to formstabilizing films. Alber et al. suggest that filaments may be formedfrom such materials by extruding alloys thereof with compatible metalswhose oxides are substantially insoluble in the molten jet. Thus, forexample, a jet of a ferrous alloy containing a small amount of a metal,such as aluminum, the oxide of which is insoluble in the jet, may beeffectively stabilized against surface tension promoted break-up,pending solidification by normal or even accelerated heat transferphenomena.

Generally, the formation of fibers and filaments by the film stabilizedinviscid spinning process is applicable to the extrusion of jets havingdiameters of less than about 50 mils. It appears that sufiicienttransfer of heat out of the molten stream, having a diameter of 50 milsor greater, even though film stabilized, is difiicult to accomplish as apractical matter to prevent break-up even when the jet is extruded intoa cooled chamber. On the other hand, when using the film stabilizationtechnique, fine diameter fibers can be adequately cooled prior tobreak-up at room temperature or greater sothat there is no necessity forelaborate cooling systems for chilling or attempting to supercool themolten jet. In order to provide sufficient jet lengths initially toprovide for film stabilization of the molten filamentary shaped jet, theextrusion velocity of jet in a given case should be such that theRayleigh parameter (Ra), a dimensionless quantity,

Ra=V\/L lies between 1 and 50, preferably 2 to 25, where V is the jetvelocity (cm./sec.), D is the jet diameter upon issue (cm.), p and a arethe melt density (gm./cm. and surface tension (dynes/cm. respectively,of the molten material. When the velocity is such that the Rayleighparameter falls below about 1.0, the jet length may be so short that itnormally cannot be adequately stabilized prior to breakup. Conversely,when the velocity of the jet is too high, breakup can be caused byaerodynamic effects.

The materials having essentially inviscid melts which are generallyemployed to form fibers and filaments are generally those normally solidmetals and inorganic non-metals having melt viscosities below about 10poises and usually a fraction of a poise. By normally solid is meantthose materials which are in the solid phase at about 25 C. The termmetals is meant to include the metals, alloys thereof, and intermetalliccompounds. The term inorganic non-metals includes the ceramics,metalloids, and salts.

Among the normally solid metals are beryllium, cobalt, aluminum,thorium, nickel, iron, copper, gold, uranium, zinc, magnesium, tin, andalloys made from such metals. Representative of the low melt viscosityceramics are alumina, calcia, magnesia, zirconia and mixtures of theseand other oxides which exhibit low melt viscosities. Metalloids, such asboron and silicon, salts such as potassium chloride and a variety ofother normally solid materials having melt viscosities below aboutpoises may be employed to make filaments and fibers by the stabilizationtechniques described hereinbefore.

A significant improvement in the extrusion of essentially inviscidmaterials is described and claimed in application Ser. No. 838,593 whichis incorporated by way of reference herein. It is recognized thereinthat the nature of essentially inviscid materials precludes attenuationof extruded molten jets by drawing as in the case of viscous andpolymeric organic materials. Yet attenuation is important in processesfor spinning materials primarily because of the difficulty in makingtrue fine diameter orifices in materials which are both substantiallyinert at high temperatures and strong enough to withstand the extrusionpressure needed to force the melt through small orifices at these sametemperatures. The problem was solved by initially directing a quantityof inert gas against the emerging jet in a direction perpendicular tothe jet path, and then causing the inert gas to flow co-currently withthe jet. It is thought that the resulting attenuation in the jetdiameter is primarily due to the viscous drag interaction of the inertgas with the jet. The degree of attentuation may be controlled by anumber of variables such as the amount of inert gas entering the systemand the geometry of an inert gas chamber which is defined by the orificeplate and a second plate known as the gas plate spaced beneath theorifice plate.

An entirely unrelated problem also noted in application Ser. No. 838,593is the disruption of the jet by materials which form in the orifice orabout the orifice exit. It is recognized that the undesired materialsare the reaction or decomposition products formed as a result of the stabilizing atmosphere contacting the jet in the orifice and/ or in theimmediate vicinity of the orifice exit. By blanketing the regionimmediately beneath the orifice with an inert gas, however, areaction-free region is maintained in and beneath the orifice therebyallowing an unimpeded extrusion of the molten materials as a jet.

Under some conditions even when utilizing the inert gas certaindisruptive effects have been noted, particularly when attempting toextrude small diameter and/ or light filaments which are trulycontinuous, e.g., 1000 feet or more. The effects appear to beparticularly deleterious under conditions necessary for the attenuationof the jet diameter to below about 50% of the orifice diameter. Suchlarge attenuations need a high co-current flow of the inert gas whichfrequently results in the jet being disrupted and in the formation ofshort fiber or staple. Often, the filaments or fibers have a wavyappearance instead of being straight. Further, some of the filaments andfibers have been found to be weakened due to the stresses incurredduring solidification.

Failure of the stabilizing atmosphere to properly stabilize the jetunder conditions necessary for large jet attenuation with the gas platehas also been observed, particularly when the stabilizing reaction ordecomposition occurs slowly or the concentration of the stabilizingcomponent in the stabilizing atmosphere is low. Under these conditions,only short, irregularly shaped fiber is formed.

When utilized denser inert gases, e.g., argon as compared to helium, ithas been noted that the adverse effects stated above are accentuated. Itis therefore desirable to use helium, which is the least dense of theinert gases. Helium, however, is considerably more expensive.

While the advantages of utilizing the process and apparatus ofapplication Ser. No. 838,593 are readily evident, it is also apparentthat it would be extremely beneficial to improve thereupon so as toeconomically permit uninterrupted extrusion of continuous filaments,particularly when attenuating to obtain filaments ha ving smalldiarneters. It is therefore an object of the present invention toprovide the above improvement and concomitantly ensure that properstabilization of the jet occurs, particularly when large attenuation ofthe jet is desired.

4 BRIEF STATEMENT OF THE INVENTION It has been discovered that thebreak-up of the jet continuity or the wavy appearance in the resultingfilaments and fibers is largely due to the high relative velocitybetween the jet and inert gas. Typically, the cocurrent velocity of theinert gas is ten to twenty times as great as the jet velocity, whenutilizing the attenuation process and appparaus of application Ser. No.838,593. The interaction between the inert gas and the inherent minorlateral deviations (bends in the jet length) and surface irregularitiesof the jet gives rise to the well-known Bernoulli effect. The lowpressure regions which form adjacent to the bends in the jet orirregular surface protrusions are analogous to the low pressure regionsabove the wing of a moving aircraft. The low pressure region's tend toincrease the amplitudes of the bends and irregularities, sometimes tothe point of disrupting the jet thereby causing the formation of wavyfilaments, stape, and/ or filaments with weakened tensile strengths.

Additionally, it has been observed that the co-current inert gas flowmay continue downstream into the zone in which stabilization shouldoccur. The inert gas then may act as a barrier or blanket to thestabilization gas, and in extreme cases, prevent proper stabilizationfrom occur ring. By removing the inert gas a short distance downstrea ofthe initial impingement point against the jet in accordance with thepresent invention, it has been found that the undesired sinous effectmay be prevented while concomitantly providing proper filmstabilization.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present inventionwhich are desired to be protected are pointed out with particularity inthe appended claims. The invention itself, together with further objectsand advantages thereof, may be best understood with reference to thefollowing description taken in connection with the appended drawings inwhich:

FIG. 1 is a vertical cross-section of a typical spinning apparatusemploying an orifice assembly in accordance with the present invention.

FIG. 2 is an enlarged, partial view of the orifice assembly of FIG. 1.

FIG. 3 is a vertical cross-section of another orifice assembly inaccordance with the' present invention.

DESCRIPTION FIG. 1 depicts a crucible 10 enclosing a quantity of moltenessentially inviscid material 11. Functionally as part of the base ofcrucible 10 is an orifice plate 12 having an extrusion orifice 13.Spaced beneath plate 12 is a gas plate 14 having an orifice or gas platethroat .15 which is aligned substantially coaxial with orifice 13.Plates 12 and 14 define a substantially enclosed chamber referred to asthe inert gas zone 16.

Beneath gas plate 14 is a plate 17 hereinafter called a suction plate,having an orifice or suction plate throat 18 which is alignedsubstantially coaxial with throat 15 (and therefore with orifice 13).Suction plate 17 and gas plate 14 define a second substantially enclosedchamber 19.

Pedestal 20 supports the entire apparatus and also defines a largecavity 21 for the stabilization of the molten jet 22.

Under positive pressure supplied to molten material 11 by an externalmeans (not shown), the jet 22 issues from the extrusion orifice 13 intochamber 16. Chamber '16 is provided with a quantity of inert gas whichis supplied under pressure through inlet port 23. The inert gas isconstrained to move laterally between orifice plate 12 and gas plate 14and thus contacts the emerging jet 22 in a direction initially normal tothe path of jet 22. This flow is in a large measure self-distributingtoward symmetrical fiow. The inert gas then flows co-currently with jet22 through the gas plate throat .15. It is thought that the reduction inthe diameter of jet 22 is primarily due to the viscous drag interactionof inert gas with jet 22.

A suction is supplied to outlet port 24 creating a low pressure zone inthe chamber 19 and thereby causing the inert gas to be stripped awayfrom jet 22 as it enters chamber 19. Simultaneously, the stabilizingatmosphere which may be introduced into the system in cavity 21 flowsinto chamber 19 under the influence of the low pressure zone andcontacts the jet for proper stabilization.

FIG. 2 illustrates the general geometrical relationship between plates12, 14 and 17 and their respective orifices. The relative size of theorifices, however, is not to scale in order to promote clarity. Gasplate throat 15 has a diameter which is greater than that of orifice 13.The diameter of the gas plate throat is below 30 times and preferablybelow times the diameter of the orifice. When the gas plate throat is astraight bore orifice as in the case of gas plate throat 15, it ispreferable that the suction plate throat be as large or larger indiameter but it may be utilized when as small as one-half the diameterof the gas plate throat. It should be recognized, however, that when thediameter of the gas plate throat is nearly that of the orifice diameter,it is necessary that the diameter of suction plate be larger. Suctionplate throat 18 is illustrated as being larger than throat 15. Arrows 25and 26 illustrate the respective paths of the inert gas andstabilization atmosphere.

As stated in application Ser. 'No. 838,593, gap 27 between orifice plate12 and gas plate 14 should be less than fifteen times the diameter oforifice 13 and preferably less than one-half the diameter of gas platethroat 15. The length of gas plate throat is maintained at less thanabout one hundred times and preferably less than fifty times the orificediameter. On the other hand, the dimensions of gap 28 between gas plate14 and suction plate 17 is not considered to be critical althoughoptimum extrusion conditions have been observed when gap 28 ismaintained between about five and ten mils. A spacing outside this rangetends to make it more difiicult to obtain the low pressure zone.

One geometrical relationship must always be observed. That is, theorifice and throats must be aligned substantially coaxial for optimumextrusion conditions. Nonalignment may cause the stream to deviate toone side of the gas plate and suction plate throats.

FIG. 3 illustrates another embodiment of an orifice spinning assembly inaccordance with the present invention. In this embodiment, gas plate 32has a gas plate throat 33 which takes the form of a tapered orifice withthe walls thereof diverging away from extrusion orifice 31 of orificeplate 30. As before, suction plate 34 with suction plate throat 35 isspaced beneath gas plate 32. Orifice 31, throat 33, and throat 35 areessentially coaxial. The inert gas is supplied to the chamber defined byplates 30 and 32 while a suction is applied to the chamber defined byplates 32 and 34.

In the embodiment of FIG. 3, it is preferable that suction plate throat35 have a diameter greater than the entrance diameter of gas platethroat 33 but less than the exit diameter. This particular constructionappears to provide a more complete stripping of the inert gas from theextruded jet. The increased stripping is belived to be the result of anumber of factors. The increasing crosssection of throat 33 results in agreater volume which the inert gas can occupy, thus causing the inertgas to become les dense about the jet. There is also a tendency for themoving stream of inert gas to follow the diverging contour of throat 33.Finally, the smaller diameter of throat 35 relative to the exit diameterof throat 33 physically blocks the passage of a portion of the inert gasfollowing the jet.

EXAMPLE I The apparatus depicted in FIG. 1, without the suction plate 17which was replaced by a spacer element, was placed in a resistanceheated melt-spinning assembly. The crucible 10 was charged with an alloycomprising 61.8% wt. lead and 38.2 wt. percent tin. The extrusionorifice 13 had a diameter of 4 mils and an aspect ratio of 1.5. Theinert gas plate throat 1 5 had a diameter of 15.9 mils and an aspectratio of 1.33. The gap between plates 12 and 14 was 8 mils. Cavity 21was open to the atmosphere.

The alloy was melted under the influence of a vacuum to a temperature of300 C. and subsequent thereto the melt was extruded through theapplication of a pressure of 20 p.s.i.g. of argon. Subsequent to theinitiation of the molten jet stream, helium was introduced into theinert gas zone 16 at the rate of 3,000 cc./min. The molten jet streamwas observed under a strobe light and appeared to have a sinusoidalconfiguration along a portion thereof. The resultant filaments had awavy appearance over their entire length which was noted to vary betweenone and two feet with an average filament diameter of 2 mils. While thejet stream had been attenuated it was impossible to produce a continuouslength filament principally due to the Bernoulli distortions of thestream and the absence of sufficient stabilizing gas vicinal to theextrusion orifice to effect complete stabilization thereof.

EXAMPLE II The experiment of Example I was repeated after the suctionplate 17 was placed in the apparatus of FIG. 1. The suction plate had athroat diameter of 62 mils with an aspect ratio of approximately 0.33.The gap between the plates 14 and 17 was 10 mils.

Subsequent to the initiation of streaming through the extrusion orifice13, helium was introduced into the inert gas zone 16 at the rate of3,000 cc./min. whereupon the jet stream responded as in Example I.Vacuum was then applied to port 24 and as the vacuum was graduallyincreased the stream was observed to be distortion free at which timeshot was produced. A slight increase in suction flow resulted in thestripping of the helium gas from the jet surface with the simultaneousincrease in the flow of the atmosphere upward into the low pressure zonewhereupon a very stable stream resulted in the production of a smooth,continuous filament having a substantially uniform diameter of 2.1 mils.

EXAMPLE III The apparatus of FIG. 1 was adapted for utilization with aninductively heated melt spinning unit for the production of filamentsfrom EC grade aluminum.

The extrusion orifice had a diameter of 12.0 mils with an aspect ratioof 4. The gas plate had a throat diameter of 40 mils with an aspectratio of 1.2. The inert gas gap residing between the orifice plate 12and the gas plate 14 was 15 mils.

The suction plate 17 had a throat diameter of 60 mils with an aspectratio of 0.6. The suction gas gap residing between the plates 14 and 17was 15.0 mils.

A charge of EC grade aluminum was placed in the melt crucible and washeated to a temperature of 800 C. under the influence of a vacuum.Subsequent to the attainment of the melt extrusion temperature, argon ata pressure of 6.0 p.s.i.g. was applied to the melt whereby streaming ofthe melt through the orifice and gas plate assembly was elfected into astabilizing atmosphere of air.

Helium at a flow rate of 5,000 cc./ min. was introduced to the inert gaszone 16 through port 33. A high velocity helium flow in passing throughthe throats 15 and 18 cocurrent with the jet stream resulted in thesinuous breakup thereof which prevented the formation of continuousfiber. After observing the disruption of the stream, a vacuum wasapplied to the exhaust port 24 which resulted in exhausting a mixture ofhelium and air at a flow rate of approximately 6500 cc./min. whereuponthe molten jet stream became steady and long lengths of aluminumfilaments were produced having an average diameter of 9.0 mils.

It is apparent from the above examples that a substan tially high rateof inert gas flow is required to effect a reduction in the molten streamdiameter along with the prevention of the ditfusion of the stabilizinggas medium upstream to the exit side of the extrusion orifice. This flowrate results in disrupting the molten jet stream such that it can onlybe stabilized under very closely controlled conditions. Rapid removingthe inert gas, however, from about the vicinity of the jet stream to theextrusion orifice obviates the effects of this high rate of flow andsimultaneously affords a simplified process for the production ofsubstantially continuous filaments.

'While the drawings and discussions herein relate to certain preferredand simplified gas plate geometries, other arrangements may be employed.Further the combination of the orifice plate, gas plate, and suctionplate may be assembled in a variety of ways. For example, the orificeplate, gas plate, and suction plate may be separate or integral membersas desired. The materials which are utilized in the fabrication of theplates should be essentially inert, each to the other, under theconditions of the extrusion process. Moreover, the materials mustnecessarily be resistant to thermal shock and to withstand the inherentmechanical stresses in the extrusion process. For example, in theextrusion of metals such as copper and ferrous alloys, it may bepreferable to use ceramic materials such as high density alumina,beryllia, and zirconia or the best resistant materials such as graphite.When extruding high temperature melts such as ceramics, graphite andmolybdenum may be employed. In low temperature extrusion processes,stainless steel assemblies have been found to perform adequately. Othermaterials and combinations commensurate with the practice of the presentinvention may also be used.

From the foregoing, it should be evident that the objects as set forthhave been obtained. Of paramount importance is the substantialelimination of the sinuous disturbances which often occur along a jetwhen fabricating fine diameter and/ or light fiber directly from anessentially inviscid melt.

Although the description has been limited to particular embodiments ofthe present invention, it is thought that modifications and variationswould be obvious to one skilled in the art in light of the aboveteachings. It is understood, therefore, that changes may be made in thefeatures of the present invention described herein which fall within thefull intended scope of the invention as defined by the following claims.

We claim:

1. An orifice assembly for the formation of fibers and filaments by theextrusion of an essentially inviscid melt as a free molten filamentarystream comprising sequentially disposed (a) a first plate having a firstorifice being substantially the size of the initial molten filament tobe formed by the extrusion of said melt therethrough;

(b) a second plate having a second orifice;

(c) a third plate having a third orifice, said first and second platesdefining a first substantially enclosed chamber, and said second andthird plates defining a second substantially enclosed chamber whereinsaid second orifice connects with said first and second chambers, saidorifices being positioned substantially coaxial with respect to eachother, said first enclosed chamber having a gap distance therebetween inthe vicinity of said first and second orifices of less than one-half thediameter of said second plate orifice, said second orifice having alength less than times the diameter of said first orifice and a diameterless than 30 times the diameter of said first orifice, said thirdorifice having a diameter equal to or greater than one-half the diameterof said second orifice;

(d) a means for supplying an inert gas to said first substantiallyenclosed chamber and into said second orifice; and

(e) a means for evacuating said second substantially enclosed chamber ofthe inert gas which has entered therein through said second orifice.

2. The orifice assembly of claim 1 in which said second orifice takesthe form of a tapered orifice having Walls which diverge in thedirection of said third orifice.

3. The orifice assembly of claim 2 in which the crosssectional diameterof said third orifice is larger than the entrance diameter of saidsecond orifice but smaller than the exit diameter thereof.

4. The orifice assembly of claim 1 in which said second and third platesare spaced apart a distance of between 5 and 20 mils in the vicinity ofsaid second and third orifices.

References Cited UNITED STATES PATENTS 2,273,105 2/1942 Heckert 18-8 QMX 2,818,461 12/1957 Gruber et a1 164-259 X 2,976,590 3/1961 Pond 164823,447,202 6/1969 Kato 18-8 QM J. SPENCER OVERHOLSER, Primary Examiner M.O. SUTTON, Assistant Examiner US. Cl. X.R.

